An instance of `cdc::topology_description` can be quite big. The vector
it consists of stores as many `token_range_description`s as there are
vnodes, and the size of each `token_range_description` is O(#shards).
Because of that, copying an instance of the type can lead to reactor
stalls. To prevent that, we introduce an asynchronous function copying
the contents on the object.
Reactor stalls were detected in the call to `map_reduce` in
`generation_service::legacy_do_handle_cdc_generation`, so let's start
using the new function there.
A similar scenario occurs in `generation_service::handle_cdc_generation`,
so we modify it too.
Unfortunately, it doesn't seem viable to provide a reproducer of said
problem.
Fixesscylladb/scylladb#24522
Backport: none. Reactor stalls are not critical.
Closesscylladb/scylladb#25730
* github.com:scylladb/scylladb:
cdc/generation: Delete copy constructors of topology_description
cdc/generation: Clone topology_description asynchronously
f95808cbe7